Matchless clone build issue

[gerry113377]

I just finished a matchless DC30 clone build from a kit I got from eBay. The amp works but there are a few issues. Issue 1: CH1 has a small amount of hum when I plug into CH1 and when I crank the CH1 volume pot all the way it crackles. If I then turn it down to about 8 the crackle goes away. There is loud guitar sound and it doesn't sound too bad though. If I plug into CH2 it sounds great no hum or crackling, but if I turn CH2 volume all the way down, and I then turn up CH1 volume, I can faintly hear the guitar and it gets louder as I turn it up, even though I'm not plugged in to CH1. Issue 2: when I switch from hi to lo power, the middle 2 power tubes start to red plate. Issue 3: this amp goes from quiet to super loud, is there any way to get a happy medium? I am attaching files Sorry for my ignorance, this is my first amp build.

[pdf64]

Issue 3 first, as it’s the most serious; don’t allow tubes to red plate.
What is the resistance between each power tube cathode terminal 3 and 0V (aka circuit common, chassis, ground), in full power and half power modes? 8 measurement results to be reported.

Regarding the volume difficulty, do you mean that it’s a ‘hair trigger’, ie very difficult to achieve a setting you want?

[gerry113377]

The reading are
full power: 300ohm, 72ohm, 72ohm, 300ohm
half power: open circuit, 130ohm, 130ohm, open circuit

Yes, it's a hair trigger that goes from quiet with no soul to full guns blazing.

[sluckey]

Your switch is not wired correctly. The half power readings are correct. But in full power all four cathodes should be connected together and the resistance reading should be 68Ω for all four.

[gerry113377]

I have triple checked all the wiring in the build so I know the switch is wired correct. But there is a possibility that the switch is bad causing the higher resistance. I will check resistance through the switch tomorrow and let you know. At least now I know what circuit to start my troubleshooting. Thanks! I have been stumped with this for months. I have spent a lot of time and money on this build and would love to get it working. Much appreciated!

[sluckey]

Just use your ohm meter to check the switch.

[gerry113377]

Since this was my first amp build, I bought some cheap used Russian power tubes from eBay. They said matched 6p14p-ev. Well, I switched them out with a new matched set and my hot plate issue is gone. As for the different OHM readings, for some reason, the resistance jumps between
60- 130ohm but everything is wired right and sometimes they all read the same, sometimes not.

As for the crackling gain issue, it only happens when the CH1 volume pot is wide open. When I back it off to 85kOHM it is ok, so I am thinking about wiring in an 85kOHM resistor between the CH1 volume pot and the terminal 7 grid of valve 2.

What do you think?

[sluckey]

I think your ohm meter is lying to you. Or ...

[gerry113377]

"or...?????" What does that mean.  When I have free time I will take a video and post the link and show you exactly what it is doing. The meter is not "lying" and working properly because when I check a resistor or potentiometer , it is spot on. And I have more then one meter just in case I feel one is "lying" I have already double checked. One is an Extech and the other is a Fluke.

I am checking resistance from Terminal 3 to the chassis, is this correct?

Thanks for the help, it is appreciated. I hope I don't sound off, but it seemed to me that you were insinuating that I could be lying. But I could be wrong about that, and if I am, I apologize. 

[sluckey]

I think your ohm meter is lying to you. Or ...

That means your meter is lying to you or... maybe operator error.

You seem to be having problems with resistance readings. First you say...

The reading are
full power: 300ohm, 72ohm, 72ohm, 300ohm
half power: open circuit, 130ohm, 130ohm, open circuit

...but in full power all cathodes are connected together through the switch, so all readings must be the same unless the switch is miswired or faulty. You say you know the switch is wired correctly so I suggest you test the switch with your meter but no response. Then you say...

As for the different OHM readings, for some reason, the resistance jumps between 60- 130ohm but everything is wired right and sometimes they all read the same, sometimes not.

And that brings us back to my last statement. None of that implies you are a liar.

So, what is it? 300 or 72, or 130, or somewhere between 60-130? What do you think?   

[gerry113377]

I will take a video and post the link. I work with electricity  almost every day at work because I fix industrial machines for a living, so we can rule out operator error. As you can see,  my meter has a lot of miles on it. But by no means am I an expert. I don't really work with capacitors, diodes and tubes and I'm not sure how tube amp circuits react when checking resistance. I have seen resistance jump here at work when the circuit has a transformer in it. That's why here at my job, I always disconnect the tranny before I test a circuit. Anyway, I'm a lot closer to getting this figured out thanks to this website and forum. Much appreciated.

[pdf64]

...I have seen resistance jump here at work when the circuit has a transformer in it. That's why here at my job, I always disconnect the tranny before I test a circuit...

That may be due to using an autoranging meter; as the meter switches through the ranges to find the most appropriate test current, the sudden change of current sets off back emf voltage spikes from the inductance of the transformer winding.
A suitable remedy is to switch the meter into manual range selection mode, if possible.

FYI, it's not an issue here, but beware that some industrial type meters have a much higher loading on the circuit under test when measuring voltage, than meters intended for electronics (which typically present a 10M load to the circuit under test).

[sluckey]

That Fluke is a fine Electrical Tester but it's not really what you need for electronics circuits. I'm not saying you don't know what you're doing. When I say operator error, I'm including stuff such as making good contact with the probe tips. We all have that problem from time to time. The probes I use have needle tips that usually insure a good connection.

There is a big capacitor parallel to the resistors you are trying to measure. That would not interfere with my Fluke 87V or most DMMs but it could very well be a factor on your Fluke continuity tester.

So, back to the beginning... You reported improper resistance readings when trying to measure the cathode resistance. Specifically you measured "300ohm, 72ohm, 72ohm, 300ohm" with the switch in full power position. The correct reading would be 68Ω and all four tubes would read the same because the switch ties all four cathodes together. You have ruled out wiring and operator error, so that only leaves a faulty switch or a lying meter. You still haven't tested the switch have you?

[PRR]

> some industrial type meters have a much higher loading on the circuit under test when measuring voltage, than meters intended for electronics (which typically present a 10M load to the circuit under test).
> That Fluke is a fine Electrical Tester but it's not really what you need for electronics circuits.

Fluke T5-1000 is
"V - Automatic selection of ac or dc voltage mode", "1 MΩ, <100 pF ac-coupled" "Resolution 1V"
"Ω - Open Circuit Test Voltage 2.4 V dc (nominal) , Short Circuit Current <600 μA"
https://dam-assets.fluke.com/s3fs-public/t5_xxxx_sieng0100.pdf

None of this matters for checking cathode resistors. However it will be 10% low voltage in a 100k plate circuit. And 1V resolution rules-out fix-bias amp checking on 1 Ohm cathode resistors (where we look for 0.030V, 30mV). That 2.5V for Ohms is quite high. (VTVM 1.5V, most DMMs 0.2V). I guess at 600uA it won't blow up even small semiconductors, but it will give different readings on 0.6V junctions than modern DMMs do.

I have no idea how it measures DCV if it is AC-coupled.....

[gerry113377]

This morning when I checked, I did get 68ohm solid (67.**) on all four terminals. Checked it again right after that and got 130ohm. Go figure. My Extech meter has 3 settings and you have to manually set it as it does not automatically switch from megohm, kiloohm, ohm. I have been using the ohm setting. I have some alligator clips I will use tomorrow so we can eliminate operator error, I don't think that is the case, but we will see. You just never know, but I have been digging in pretty hard with the tips when I have been checking it. As for the fluke meter that I used (in the picture). That particular meter only has one setting (ohm) and does not auto switch and I still had the readings jumping around. I should clarify that when I say jumping around, I mean that I get different reading each time I check it (pull the tip on and off), not that it jumps around with the tips still on. Each time I pull the tip off and back on, the reading can range between 60ohm-300ohm.

I do have a question when testing this circuit:

1. Should the tubes be in or out, does it matter? Both power and preamp tubes in or out?
2. Is there anything that needs to be disconnected before checking the circuit?

Thanks,
G

[sluckey]

You still have resistance measuring issues. Surely you have the power turned off while making in-circuit resistance checks. Tubes in makes no difference. As I said in my last post, that big ole cathode bypass cap 'may' be affecting your readings. That 2.5V from your Fluke tester could very well be charging that cap and throwing off the resistance readings (just a WAG). Disconnect the cap to be sure.

I think you are through checking those cathode resistors. The only reason it was ever suggested to check the cathode resistance was because you said the two inner tubes redplated when in low power. Since you no longer have redplating issues I think you're done with this check. We would have been done long ago except for your whacky measurements. 

[gerry113377]

I agree, the red plate problem has gone since I changed the tubes. And yes, power cord was unplugged and caps discharged before checking measurements. I did forget to mention I have 2ohms through the switch. Meter charging the cap is great information for future toubleshooting.

As for the next issue with the static crackling sound when I crank the CH1 volume all the way up, I will leave that for another day because I want to post a video first so you know exactly what it's doing.

Thanks,
G

[shooter]

information for future troubleshooting

future 
self biased;
I ALWAYS verify the cathode R and g2 R prior to installing new tubes.  ANY variation, 68 measures 75, I will, and have eatin the cost to replace just so I know it's right.

your PS "tip", Sluckey nailed it, pretty useless.
my test;
wired fully, unplugged, discharged, I ohm the B+ just after rectifier, other lead to chassis main ground. I expect many thousand ohms, charging.  discharge, repeat.
NO tubes, fire up measure all possible VDC at socket, other lead at main chassis ground.  Good; preamp tubes in, re-measure vdc.  Good, PA tubes in.

[sluckey]

your PS "tip", Sluckey nailed it, pretty useless.

Hmm, I think that was a different thread. 

[shooter]

 
no more multi tasking 

both threads were deep in resistance 

[gerry113377]

Okay I posted a video of what my amp is doing. Starting at time stamp 2:32 is the sound that it's making. The rest of the vid is me trying to explain the issue

CH1 has a small amount of hum when I plug into CH1 and when I crank the CH1 volume pot all the way it crackles. If I then turn it down to about 8 the crackle goes away. There is loud guitar sound and it doesn't sound too bad though. If I plug into CH2 it sounds great no hum or crackling, but if I turn CH2 volume all the way down, and I then turn up CH1 volume, I can faintly hear the guitar and it gets louder as I turn it up, even though I'm not plugged in to CH1.

[shooter]

what do you hear pluggin headphones into your FX hole

[gerry113377]

Sorry, I don't have headphones. Are we trying to eliminate the effects loop as the  problem? I can jump send 1 into return 2 and send 2 into return 1. Let me try it when I get home from work. Unless you are meaning something else. Is there a way to troubleshoot it without headphones?

[shooter]

Is there a way to troubleshoot it without headphones?

sure, a scope is best for troubleshooting the signal path 

the FX is a convenient place where the signal comes OUT for comparison.

[sluckey]

Your FX loop is located between your preamp and power amp. If you can connect the preamp out into another power amp you can find out if the problem is in your preamp or power amp. Just a divide and conquer troubleshooting technique.

BTW, there should be no interaction between channels like you are experiencing. I suspect a wiring error.

[gerry113377]

I created an open circuit on both FX channels that's only closed when I use a jumper. I also remove tubes. Here is the new video.

&t=21s

[shooter]

my crt only displays ascii txt
did that fix it?

[gerry113377]

Sorry, I forgot to add that I quadruple checked the wiring again and I did find that the output transformer P2 green and P1 yellow are swapped but I don't see how that would make any difference. Also I am getting 2Rohm between the Channel 2 FX send and ground but maybe that is normal.

[gerry113377]

It was suggested to me that the Phase Inverter CAPs could be wrong. The original DC30 has .01uf caps instead of .1uf. Could this really be the problem?

[sluckey]

Also I am getting 2Rohm between the Channel 2 FX send and ground but maybe that is normal.

That's normal ONLY if Channel 2 volume pot is set to zero. Otherwise it's abnormal.

It was suggested to me that the Phase Inverter CAPs could be wrong. The original DC30 has .01uf caps instead of .1uf. Could this really be the problem?

No.

[gerry113377]

Broken lead on the CH1 FX send 100k resistor.

[nandrewjackson]

Did fixing the broken lead fix the issue, or did you just find it and post it here?


I'm at a loss. I worked for a small amp startup 5 years ago, they were making vox/matchless clones. Every now and then we'd have a silly ghost like that to chase down.

[gerry113377]

This amp has several issues. This broken resistor lead was the issue with the volume going from quiet to super loud with no in-between. Now I have a smooth roll on the master volume.

The amp came with some pretty cheap shielded wire used for all the input, FX jacks. The next step is to replace all that CHEAP shielded wire. As this is my first build, I am still learning all the mistakes I did, as this is totally different than industrial electronics, so I need to "forget about what I know" about wiring things. For instance, I did not wire the 68k resistor to the grid pin but instead wired it to the input jack. Also, it appears on isolated input jacks, it is recommended to connect the input jack ground lug to the ground with a .01uf capacitor. I know all my issues are in the preamp so it's just a matter of time before I get it sorted. Little bit by little bit.

[gerry113377]

When I disconnect the  CH1 treble pot wiper, all the hum stopped. I guess I am going to leave it like that for now until I have time to rewire and replace some components. At least I have half an amp. It sounds great on the EF86 side. Thank you all for your help.

[nandrewjackson]

It's nice to hear that you're making progress on your amp !